For the first time, scientists film a fish thought in action

Have you ever looked into the glassy eyes of a pet goldfish in a bowl and wondered what’s going on in that little critter’s noggin? Wonder no more. Scientists have for the first time captured a fish thought in action as it sparks through the brain, according to a new study published online Thursday in the journal Current Biology.

A team of Japanese scientists, using an ultra-sensitive technique that could become a handy tool in studies of perception, watched a zebrafish’s brain light up as it watched a tasty-looking paramecium swim around it.

Zebrafish are transparent while they’re still larvae, making them excellent test subjects – you can see what's going on inside their bodies, and in their brains. Still, such activity can be challenging to track.

To do so, the researchers honed a technique that uses genetically modified fish whose brains fluoresce in certain parts wherever calcium ions – which help deliver signals in the brain – crop up. They set loose a tasty little paramecium – single-celled critters that make good baby food for zebrafish – and watched the fish's brain light up in a region known as the optic tectum, which contains neurons linked to the visual system.

When the paramecium swam to the right, the zebrafish brain’s left hemisphere lighted up. When it swam over to the left, the right hemisphere lighted up. This makes sense, given that in vertebrate animals the brain-body connection is crisscrossed: The right side of the body is connected to the left side of the brain, and the left to the right.

The researchers were able to see the brain work in remarkable detail. In one test during the study, they had the zebrafish watch a screen with a moving dot, and saw individual neurons light up in response to particular directions – up, down, left or right.

And the researchers found that the zebrafish brain only lighted up when the paramecium was moving; when it was still, the optical tectum remained dark.

They also found that when a zebrafish tried to chase the paramecium down, a more frontal part of the tectum started sparkling. That part of the tectum, the researchers surmised, must be linked to “eye convergence” – when both eyes move inward to focus on prey – as well as approach swimming.

“Our system should enable real-time imaging of genetically tagged neurons in the brain that may control behavior and locomotion,” the authors write.

This method could help scientists interpret animals’ behaviors, and find exactly where aspects of cognition like learning, memory and different emotions are based in the brain, according to the authors. And it could even help researchers quickly figure out if a drug they’re testing is effective or not.